I
get my goodies at Amazon
and Adorama.
It helps me publish this site if you get yours from those links, too.

INTRODUCTION

Center
filters are used with professional large-format wide-angle lenses
to correct the natural darkening of the sides of the image. Center
filters are dark in the center and clear on the outside. They aren't
needed with SLR cameras.

Light
falloff is the darkening of an image's corners. It
happens with wide lenses because the light has to travel much farther
to reach the corners of the image than the center.

I'm
going to explain the difference between large format lenses and SLR
lenses. Then
I'm going to explain the problems with each related to light falloff.

Then
I'll explain center filters and how to use them. If you don't
care about the technology feel free to skip straight to the usage
recommendations at the end here.

Retrofocus
SLR Lenses

The
SLR cameras most familiar to amateurs need special retrofocus
designs to keep the back of the lens away from the swinging SLR mirror.
Before retrofocus lenses were invented by Angenieux in the 1950s
there was no way to use short lenses on SLR cameras.

Retrofocus
lenses use a strong negative front section with a positive
rear section. This gives a wide angle while keeping the back
of the lens much further away from the film.

This
eliminates most falloff, since the greater distance from the film
helps equalize the distances the light travels between center and
corner. Other clever design tricks eliminate the rest of the falloff,
so falloff is something very foreign to most amateurs. Just look
through the front of any SLR ultrawide lens and you'll see the clear
center "eye" of the lens actually look bigger as you look at it from
an angle!

Large Format Wide Angle Lenses

The
reason we don't use retrofocus lenses on non-SLR cameras is because
everything else about retrofocus designs are worse then ordinary
short lenses. Retrofocus designs are much heavier with their huge
front negative elements. Retrofocus designs also create barrel
distortion which lens designers attempt to correct with varying
degrees of success. These designs also have a harder time of staying
sharp in the far corners. This is why rangefinder cameras like Leica
M and the Mamiya 7 and Mamiya
6 have
such an easy time of making excellent, compact wide lenses.

This
is also why it's normal for lenses on any non-SLR camera to have
falloff. The wider the lens, the more falloff. It depends
on angle, not focal length or format.

Falloff is easily corrected with a filter, while there is no trivial
optical way to fix a lack of sharpness or distortion.

The
Technical Side

There
are actually four reasons the sides get darker. That's why the law
is called cosine ^ 4. Cosine is the trigonometric function which
is equal to 100% at 0 degrees and drops to 0% at 90 degrees. The
four reasons for darkening of the corners are:

1.)
Distance. It's longer for the light to travel form the center of
the lens to the side of the image than the center.

2.)
Distance again. Light falls off as the square of the distance, since
at twice the distance the light is spread across four times the area.

3.)
Angle at the film. At the corner the light only grazes the film at
an angle, which spreads the light over a larger area.

4.)
Angle through the aperture of the lens. Straight on you see a circle.
As you get to the side you only see a smaller ellipse.

All four of these vary as the cosine of the angle of
light through a conventional lens. Retrofocus lenses avoid most of
this since these apply to light coming out the rear of the lens, and
the longer back focus straightens the angles involved.

At
45º off center (a 90º field of view) we
have (cos 45º) ^ 4 = 0.71 ^ 4 = 0.25, which is two stops lost.

This
very quickly starts to get very bad as the angles approach ultra-wide,
while remaining negligible for normal and tele lenses.

The Cure

Center
filters simply are dark in the middle and clear at the sides. Put
one on your wide angle lens, add as much exposure as the center is
dark, and you now have even exposure throughout your image. Most
center filters are 1.5 or 2 stops darker in the center.

I
prefer the wide angle lens' effect of darkening the sides. Ansel
Adams deliberately added this effect to many of his images by burning
the edges of his prints. See Ansel's book
"The
Print." This artistic effect emphasizes the central point
of the image and helps keep the viewers' eyes from wandering off
the image.

It's
easy to see the effect of a center filter on a 65
mm lens on a 4
x 5" camera if
you photograph blank walls. It's like night and day with and
without the filter.

The
effect is subtle even with a 65 mm lens for real subjects. This
is because of the great range from light to dark that we take for
granted. A stop or two on the sides is actually quite subtle unless
you're photographing blank walls.

This
is one
of my most famous images. It was made
with a 65 mm lens on a 4 x 5" camera and no center filter. I
deliberately avoided using my center filter because I wanted to emphasize
the center. I even added a three-stop grad, dark on top, to dim the
sky! There must be five stops more exposure in the center of the
image compared to the top. This is what gives the wall its glowing
effect so critical to the image. Needless technical perfection is
a bad thing in creative art.

Because
of this, and the loss of usually two stops and more stuff to carry
around, I rarely bother with center filters.

The
only time a center filter makes sense to me is with my 47 mm lens
on a 6 x 12 cm camera. The extreme angle, 113º at
full 16 mm of rise, computes to only 9% of the light, or 3.4 stops
of darkening in the far corners. You can see that! I love the 4x (2
stop) center filter Schneider makes for my 47 mm XL lens. You can
use this filter without any mechanical vignetting, and in fact, I often
use another 67 mm screw-in filter between the center filter and
the lens with flawless results.

I've
never bothered with a center filter for my 75 mm and longer lenses
in 4 x 5." In 6 x 7 cm my Mamiya 43
mm lens
gives a similar angle of view and the same falloff as the 75 mm lens
on 4 x 5.

How
to Use a Center Filter

I
do all my composition, movements and focusing without the filter.
When I'm all ready I screw it on and make my exposure.

I
determine exposure as I usually do, and add the appropriate exposure
compensation for the filter. This is usually 1.5 or 2 stops.

I
love to use colored filters screwed into the front of my lens. If
you're screwing a center filter to the front of your lens you reduce
your options.

You
may have to use a gel or balance your color filter on the rear of
the lens. Remember that focus changes if you put a glass filter on
the rear of your lens.

if
you're rich you can use a much larger screw in filter on the front
of the center filter. Center filters have much larger front threads
than rear threads. My 67 mm center filter for my 47XL has 86 mm front
threads!

Personally
I often cheat and simply add the colored screw in filter between
the lens and center filter. Usually you'll get nasty vignetting and
screw up the correction, however I get away with it with my 47XL
lens since if has such enormous coverage. It's rare that you can
get away with this.

If
you insert a second filter between the lens and center filter you
run the risk of having the sides of the image slightly too light.
This is because the sides of the image now look through a lighter
part of the center filter closer to its edge, giving the potential
for over correction of falloff.

As
all photography, try anything and everything at home before
you go shoot something critical.

When
I'm crazy enough to use a center filter I'm crazy enough to want
to use exactly the right one. That means one supplied by the lens
maker. As the chart at the link above shows you Schneider
makes a plethora of center filters, even with the same density and
filter size. Each lens needs something a little bit different.

I haven't tried the generic ones made by Hoya and Heliopan.
When I've looked at prices I didn't see much difference, which also
helped make up my mind.

By all means try Hoya and Heliopan and they are probably
fine, but if you're picky enough to worry about this then I'd suggest
the one made by your lens maker.

Remember
that I usually don't bother with a center filter unless I'm using
over about 90 degrees of view.

How to Tell a Good Center Filter

The
worst way is to go shoot a wall and look for perfectly even illumination.
Center filters aren't designed for complete correction out to the
very last millimeter of image circle. If they were you'd have three
or four stops of loss. Having this dense a filter would lead to
even more problems. A center filter corrects most of the falloff,
but not all.

The
best way to tell if you have a good one is to make comparisons and
ensure that it doesn't affect the color rendition. It's difficult
to make a truly neutral ND filter. Many times there will be a slight
color cast. Just try out your filter and be sure there's no objectionable
color casts.

Also test to see what filter factor works well for you.
Personally I seem to shoot my 1.5 stop filters with a 2 stop correction,
which also eases exposure calculations.

Why
Center Filters are so Big

They
are usually made of thick glass. They are used on very
wide angle lenses. The angle is large and the light has
to travel through the thick glass. The front of the filter
has to be bigger than the rear thread to avoid mechanical vignetting.
Otherwise you'd see the sides of the filter as black corners in your
image, which would defeat the entire purpose.

Why
Center Filters are so Expensive

Center
filters cost a few hundred
dollars each! They are expensive because it's very difficult to make
a filter which is both perfectly neutral and has a uniform and correct
feathering of density from the center to the sides. These cost about
$350 because they are made in one of two ways:

1.)
Take two pieces of glass, one clear and one solid neutral density.
Grind them each as if they were lens elements to be cemented together.
Grind the clear one concave and dark one convex. Now glue them together.
This gives you a dark center and clear sides.

2.)
Sputter a metallic coating on the center of a piece of glass. The
coating must be both perfectly neutral and spread with the perfect
densities towards the sides, as well as be perfectly round. Good
luck! That's why these cost $350.